Encoders are often used in environments where they are subjected to significant vibrations, and often in a heavily contaminated environment. The encoder may, for example, be used to generate the position values of shaft rotation in a machine. For example, the encoder may be attached to a rotating shaft in the machine. The encoding disk is caused to rotate as the shaft rotates, and the encoder casing and detector are firmly affixed to the casing of the machine.
Conventionally, an optoelectrical state encoder is used, which includes an encoding disk that has an optically readable pattern. The disk pattern is read by one or more sensor heads having at least one detector, which each deliver an electric signal in relation to the amount of light that is received in the detector, so that movement of the encoding disk in relation to the detector will be indicated by changes in the electric signal.
The encoding disk includes a periodic pattern, such as a plurality of reflective and non-reflective fields of mutually the same size, for example. When the encoding disk is turned or rotated, the change between and non-reflective fields can be detected and a change in angle thus determined.
Besides the use of optical/reflective encoders, there are also encoders for magnetic and inductive scanning.
However, when the shafts have very large diameters, such conventional encoders meet their limitations due to the need for specialized mounting, such as toothed belt arrangements, that are complicated and expensive. In addition, conventional encoders cannot be used for shaft ends that are inaccessible or for hollow shafts. Recently, magnetic encoders have been introduced in the market, including a magnetic tape that can be installed on large shafts more easily and inexpensively compared to conventional encoders. When a magnetic tape is used, a solid unseparated carrier ring is installed on the shaft, which carrier ring supports the magnetic tape. A sensor head for reading the magnetic tape is arranged with a small air gap close to the magnetic tape on the carrier ring that is arranged on the shaft. There is also similar equipment involved in magnetoresistive, optical/reflective and inductive scanning, including also the use of a sensor head for reading/scanning towards a carrier ring that is arranged on a shaft.
One problem with the use of magnetic, optical/reflective, and inductive encoders is still that it is difficult or even impossible to install the encoder equipment, and especially the carrier ring on a shaft. In some machines and applications, such as in wind turbines of a wind power plant, there is very little space for installations of additional equipment. The installation of an unseparated carrier ring on a shaft is dependent on the sequence of mounting of, for example, a wind turbine. In, for example, wind turbines, etc., there is also a need for keeping the weight as low as possible. Consequently, it is desirable to use light weight materials, such as light metals, for all additional equipment. When a carrier ring is mounted on a shaft in a machine, the ring has to be heated in order to mount the ring onto the shaft. A carrier ring mounted onto a shaft is screwed to a flange of the shaft. However, a shaft is often made of steel and when a carrier ring is installed by heating on the steel shaft there will be subsequent difficulties with alignment and straightening of the ring on the shaft due to different coefficients of thermal expansion of steel compared to a light metal. The problem with high stress still remains if there are materials involved having different coefficients of thermal expansion for the carrier ring and the shaft, although the carrier ring is fixed to a flange. It is important that the installation of the ring does not produce too much stress. It is a major problem and time consuming to adjust a conventional carrier ring when mounted on a shaft with conventional methods.
Usually the shaft has a form defect, for example, an oval shape or a polygon shape in a cross section of the shaft. Hence, the mounted carrier ring must be subsequently adjusted when mounted in place on the shaft, taken the form defect into consideration. With the installation of the carrier ring by heating onto the steel shaft according to conventional methods, and fixing the carrier ring to the flange of the shaft, the subsequent adjustment work to compensate for the form defect is time-consuming and troublesome.
In addition to the form defect, a position defect due to eccentricity of rotation between the carrier ring and the shaft is often present and that needs to be adjusted after mounting the carrier ring in place on the shaft. To correct for both a form defect and a position defect in an encoder, having a carrier ring installed in its position on a shaft, is complicated.
PCT International Published Patent Application No. WO 2011/018330 describes a pole wheel arrangement for a rotary encoder, wherein the pole wheel includes pole wheel segments.